Chemical Control of Valence Tautomerism of Nickel(II) Semiquinone and Nickel(III) Catecholate States

Abstract

Transition-metal complexes with dioxolene ligands have been attracting much attention because of their characteristic ligand-localized redox reactions among the three redox states of quinone (Q), semiquinone (SQ), and catecholate (Cat) in addition to metal-centered redox reactions.[1] Interest has especially been paid to interconversion of valence tautomers in the metal-dioxolene framework.[1] Valence tautomers are characterized by displacement of electron density to one redox center or the other, for example, M

Radical ligand-containing single-molecule magnets

“…Here, the close proximity of energies between frontier orbitals for the metal centers and radical ligands not only leads to strong coupling, but can also induce electron transfer between the metal and the ligand in these complexes. This electron-transfer process is known as valence tautomerism, and is currently an active area of research in the field of molecular magnetism [52,[134][135][136][137][138].…”

Mixed‐Ligand Copper Complexes with 8‐Methylthioquinoline and Triphenylphosphane or theo‐Semiquinone/Catecholate Redox System

“…[4] (1) Valence-tautomer (or redox isomer) equilibria are not only of interest for the enzymatic mechanism of copperdependent amine oxidases; [3] (1) has also been implicated in catechol-enhanced Fenton processes for wood decay. [5] In general, valence-tautomer equilibria [6] involving cobalt, [7] manganese, copper, [4,8] nickel, [9] and iron [10] have been discussed with respect to potential applications in molecular electronics ("switching"). [6][7][8][9][10][11] While thioether (methionine) coordination to biological copper(i/ii) is observed for electron transfer proteins [1,2a] and enzymes, [12] there has also been a recent report describing N-S five-membered ring chelate binding of copper in methanobactin.…”

“…[5] In general, valence-tautomer equilibria [6] involving cobalt, [7] manganese, copper, [4,8] nickel, [9] and iron [10] have been discussed with respect to potential applications in molecular electronics ("switching"). [6][7][8][9][10][11] While thioether (methionine) coordination to biological copper(i/ii) is observed for electron transfer proteins [1,2a] and enzymes, [12] there has also been a recent report describing N-S five-membered ring chelate binding of copper in methanobactin. [13] Extending our previous approach [4] to other N-S chelate ligands we have focused on 8-methylthioquinoline (MTQ), [14][15][16] which contains an azine (pyridyl) nitrogen donor instead of the more basic but less π-accepting azole (imidazole) N in mmb.…”

The Geometric and Electronic Structure of a One‐Electron‐Oxidized Nickel(II) Bis(salicylidene)diamine Complex

“…Given favorable energetics, valence tautomerism can occur through variation of the ligand field or temperature. [2,[5][6][7][8][9] In particular, much recent interest exists in nickel(II) bis(salicylidene)diamine complexes (Scheme 1), [6][7][8][9][10] ) forms.Although extensive spectroscopic and electrochemical data exists for such oxidized complexes, structural data is lacking. Herein we report the X-ray crystal structure of a Ni IIligand-radical complex (1 + ), which has a contracted coordination sphere relative to its neutral analogue 1.…”

“…[1] In an effort to understand the intricacies of the interaction of metal ions with organic radicals, many transition-metal complexes with one or more organic radical ligands have been studied. [2][3][4] Depending on the relative energies of the redox-active orbitals, metal complexes with proradical ligands can exist in a limiting description as a metal-ligand-radical (M n+ (LC)) or a high-valent metal complex (M (n+1)+ (L À )). Given favorable energetics, valence tautomerism can occur through variation of the ligand field or temperature.…”